Taxonomy module provides the function
taxonomy_get_term_by_name(), which lets you retrieve an array of term objects matching a given term name.
However, I've found several instances where I'm using either only one free-tagging vocabulary, a collection of hand-crafted vocabularies, where I know terms don't have repeats. And I'm only interested in grabbing the term ID of the first result so that I can monkey with it elsewhere in my script.
So here's a new function,
var_dump(taxonomy_get_tid_by_name('Drupal project')); // Found, one result... returns that term's ID.
var_dump(taxonomy_get_tid_by_name('adsasdasd')); // Not found... returns FALSE.
var_dump(taxonomy_get_tid_by_name('Modules')); // Found, two results... returns the *first* term's ID.